An engine control system of a vehicle controls the delivery of air and fuel to a cylinder of an engine. The mixture of air and fuel is combusted within the cylinder to generate torque. More specifically, combustion of the air/fuel mixture releases thermal energy that drives pistons within the cylinders to power the vehicle. A fuel injector associated with the cylinder provides the fuel of the air/fuel mixture. The amount of fuel provided by the fuel injector is based on an amount of air provided to the engine for a target torque.
One way to reduce emissions from an engine involves recirculating exhaust gas into the combustion process. For example, exhaust gas recirculation (EGR) may be used in a diesel engine. EGR decreases exhaust emissions but tends to make combustion less stable. Providing a small pilot injection quantity (or “shot”) prior to a main fuel injection shot may help to stabilize combustion when EGR is used. The amount of fuel in the pilot injection shot is typically less than the main shot. The amount and timing of the pilot injection shot is usually based on a calibrated amount for the engine. Deviations from the calibrated amount and timing of the pilot shot may reduce its effectiveness in aiding combustion and decreasing exhaust emissions.
Fuel injectors may be operated by associating a fuel injector energizing time with a fuel injection amount. The actual amount of the resulting pilot shot or main shot may be a function of the fuel injector construction and the pressure of the fuel delivered to the fuel injector from a fuel rail. A fuel injector calibration map may be created by performing a bench test. Actual injection amounts may be measured and stored for different injector energizing times at different fuel rail pressures. When a control system of the engine commands a particular fuel amount to be injected, the calibration map may be consulted to return a fuel injector energizing time for the fuel rail pressure. Any values not included in the calibration map may be interpolated from the calibration map.
Fuel injectors and engine systems may have variations such that a calibration map does not precisely match fuel injection characteristics in a particular vehicle. Fuel injectors may also be faulty or may degrade over time (i.e., injector aging). Injector aging may result in injection of fuel quantities different from the expected quantity for a particular energizing time and rail pressure. Pilot injection may involve quantities of fuel at the low end of the fuel injector operating range, particularly when the fuel rail pressure is high. The fuel injector may have a minimum energizing time, and at high fuel rail pressures may not be able to deliver a small amount of fuel desired for a pilot shot.